It is entirely possible that labels encourage artists to record secular songs because overly religious songs are relegated to very limited airplay and thus exposure.
Keep in mind record labels get no royalty from radio airplay. Also keep in mind there's an entire radio format built around religious music. So if the goal it to get airplay, they could aim that way. I'm sure there's no aversion to playing religious Christmas music on K-Love and Air 1.
I do take issue with the idea that radio doesn't play these religious songs because no one wants to hear them before 12/24, but they want to hear "Winter Wonderland" for eight weeks straight.
The real problem with the religious songs is they tend to be ballads and use non-colloquial language. I've had conversations with artists on this subject. They've tried to update the language in these songs and make them more conversational, but that's controversial with the religious purists. But we've covered the subject of tempo in radio programming at other times of the year, and those rules don't go away in December. The fact is that the secular songs are all cheerful and uptempo, and that makes them better for radio.